Optical probes with small footprints are employed in applications where interrogating materials and media with conventional means become difficult because of restricted access and/or environmental hazards. For example, such optical probes can be utilized for probing oil fields (see Nakstad et al., “Probing oil fields”, Industry Perspectives, Oil and Gas Applications, Technology Focus: Optical-fibre sensors, Nature Photonics Vol. 2, No. 3, pp. 147-149, (2008)), nuclear reactors (see PCT Pat. Appl. Pub. No. WO/2012060563 OPTICAL FIBER PROBE FOR MEASURING PH IN NUCLEAR REACTOR COOLING SYSTEM AND PH MEASURING SYSTEM USING SAME) or contaminated soils (see Ghandehari et al., “Near-Infrared Spectroscopy for In Situ Monitoring of Geoenvironment”, JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING Vol. 134, No. 4, pp. 487-496, (2008)). In the medical field, optical fiber endoscopes constitute tools of prime importance for clinicians, as internal organs and tracts can often be readily accessed without surgery for diagnostic and treatment purposes.
Optical probes can exploit various interaction mechanisms or principles involving light to extract information from the probed object or medium. For some types of probes, excitation photons impinge on the object and interact with it so that secondary photons are created within the object or at its surface. Several light-matter interaction mechanisms can lead to the generation of secondary photons, such as Raman scattering, harmonic generation and fluorescence, to name just a few.
Useful information about the chemical composition, function and structure of the probed object or medium can be extracted from these secondary photons, once collected by appropriate means. One such means can involve a device comprising one or several optical waveguides, such as an optical fiber, wherein the secondary photons are collected and transmitted to a detection system. In particular, a dual-clad fiber (DCF) offers a well-suited, compact solution as both excitation and secondary light can be transmitted by the same fiber. In this case, the excitation light and the collected secondary light travel in different waveguides that are part of the same optical fiber.
An example of optical fiber probe using a DCF has been described by Veilleux et al. (see Veilleux et al., “Design and modeling of a prototype fiber scanning CARS endoscope” Proc. SPIE Vol. 7558, paper no. 75580D, (2010)). This probe, an imaging endoscope exploiting coherent anti-Stokes Raman scattering (CARS), could find numerous medical applications. CARS involves parametric processes initiated with two optical excitation signals having different wavelengths, namely a “pump” lightwave and a “Stokes” lightwave. A well-known limitation of CARS endoscopy comes from the generation of a contaminating background signal at the same wavelength as that of the CARS signal. The contaminating signal is generated through four-wave mixing (FWM) interactions involving the excitation photons as they propagate along the optical fiber. Appropriate means for managing this contaminating signal are desirable as otherwise part of it can be eventually collected and superposed to the CARS signal, thus leading to a degradation of the CARS signal-to-noise ratio (SNR). In the case of an imaging application, this contaminating background light limits the contrast of the images.
Various approaches have been proposed to mitigate this contaminating signal but, depending on the intended size of the overall probe, these approaches may not necessarily be practical. For example, the efficiency of the FWM nonlinear parametric process that gives rise to the contaminating signal at the CARS wavelength is polarization-dependent. The use of orthogonal polarizations for the pump and Stokes lightwaves can reduce the generation of the undesirable signal. The cross-polarization method implies that the polarization direction of one of the output lightwaves (either the pump or the Stokes) be rotated so that both lightwaves get co-polarized when incident on the probed tissue or object. The miniaturization of the probe may then become a challenge.
The use of a double-core fiber is another way to reduce the FWM generation. The pump and Stokes lightwaves propagate in two separate cores with a minimum of overlap between the corresponding transverse modes. However, the double-core design has its own limitations: the injection of the pump and Stokes lightwaves into separate cores gets more complex and the lens design at the output end of the fiber must ensure good spatial overlap of the light beams on the tissue under examination, thus resulting in additional constraints to account for in the course of the design phase.